In surface coating systems, compressed air is made available either directly from a compressor or from a compressed air network, and is employed for fluidizing, for conveying and for atomizing coating powder. In coating systems used to apply liquid materials, compressed air is used for atomizing coating liquids (e.g. paints and lacquers). In both powder and liquid systems, pressure-reducing valves are provided in order to regulate the compressed air supplied from the network or compressor, with common operating pressures falling generally in a range of 8 through 12 bar.
Compressed air is available nearly everywhere, and represents an extremely cost-beneficial pressure medium. Intensive investigations, however, have shown that compressed air also has numerous disadvantages when used in certain surface coating systems. For example, compressed air entrains water and oil residues. The presence of water and oil in the coating material being applied to the work piece may result in unacceptable coating results. Consequently, water separators and de-oilers are utilized, considerably increasing the operating costs of the coating system. It is also significant that the temperature of the compressed air in such systems either corresponds to ambient temperature, or is slightly above ambient temperature, due to the compression process. The ionization capability of air is proportional to its temperature. As a result, comparatively warm compressed air employed for atomizing the coating material in electrostatic coating systems becomes highly charged, thus degrading the efficiency of charging the powder particles or liquid droplets. Although it is possible to cool the atomizer air with cooling units, such practice is relatively expensive. Further problems with cooling units arise in the need for insulation to maintain lower air temperatures, and in the increased bulk and weight of cooling units integrated into manual spray guns.